Description: The long term objective of this proposal is to develop a quantitative means for predicting the delivery of semivolatile organic contaminants from hazardous waste sites via nearby streams. To model the exchange of such chemicals between the stream water and the immobile bed sediments. It is proposed that measuring the penetration of excess Th234 and Pb210 into the bed of a stream will reveal the depth of sediment which is in contact with the overlying flow on time scales of weeks (Th234) and which is accumulating over decades (Pb210). Coupling this knowledge with the sorption and diffusion behavior of toxic organic chemicals will enable modeling bedstream exchange. By performing a series of mass balance studies on a particular stream system (the East Drainage Ditch/Aberjona River passing two Superfund Sites in Woburn, MA), the researchers expect to see whether chemical degradation processes are necessary to close the mass balances for (a) short term field studies at different stream reaches and occurring at different seasons, and (b) a longer term monitoring spread over several years. Should "sinks" be needed for particular semivolatiles to explain the field data, they will apply some "probe" compounds to the stream in an effort to calibrate that site for certain photochemical (e.g., 2,5- dimethyl furan to assess the abundance of the transient oxidant, singlet oxygen) and biochemical processes (e.g., toluene to assess microbial oxidase activity).